Considerable attention has focused on stem cells and their uses in a range of therapies. The availability of somatic stem cells from adult tissues would greatly contribute to cell replacement therapies such as bone marrow transplants, gene therapies, tissue engineering, and in vitro organogenesis. Production of autologous stem cells to replace injured tissue would also reduce the need for immune suppression interventions.
Somatic stem cells, also known as adult stem cells, are stem cells derived from adult tissues, in contrast to other sources of stem cells such as cord stem cells and embryonic stem cells, which may originate from a variety of sources of embryonic tissue. Somatic stem cells are particularly attractive for a range of therapies in light of the ongoing controversies surrounding the use of embryonic stem cells.
Somatic stem cells possess the ability to renew adult tissues (Fuchs and Segre, 2000). Cell growth is a carefully regulated process that responds to the specific needs of the body in different tissues and at different stages of development. In a young animal, cell multiplication exceeds cell loss and the animal increases in size; in an adult, the processes of cell division and cell loss are balanced to maintain a steady state. For some adult cell types, renewal is rapid: intestinal cells and certain white blood cells have a half-life of a few days before they die and are replaced. In contrast, the half-life of human red blood cells is approximately 100 days; healthy liver cells rarely die, and in adults, there is a slow loss of brain cells with little or no replacement.
Somatic stem cells may also play an important role during aging. As an animal ages, cellular changes that occur in tissues are also likely to reflect alterations in the number and function of somatic stem cells. One model which has been proposed is that alterations in a cell's DNA reflects the relative age of the cell. These alterations could include stable covalent base modification (e.g., methylation) or poorly repaired forms of oxidative and other chemical damage (production of hypoxanthine and xanthine bases via deamination), as well as base pair errors introduced during replication. Accumulation of these defects over time would eventually lead to declines in stem cell number and function to the ultimate demise of tissue function and life. Thus, it would be desirable if there was a way to counter some of the aforementioned effects of aging to rejuvenate aged tissues.
Beyond their potential therapeutic applications, homogenous preparations of somatic stem cells would have another important benefit, the ability to study their molecular and biochemical properties. The existence of stem cells in somatic tissues is well established by functional tissue cell transplantation assays (Reisner et al, 1978). However, their individual identification has been difficult to accomplish. Even though their numbers have been enriched by methods such as immuno-selection with specific antibodies, there are no known markers that uniquely identify stem cells in somatic tissues (Merok and Sherley, 2001). Secondly, somatic stem cells are often present in only minute quantities, are difficult to isolate and purify, and their numbers may decrease with age. For example, brain cells from adults that may be neuronal stem cells have only been obtained by removing a portion of the brain of epileptics, not a trivial procedure.
Thus, there is a need to develop simple and reliable methods for the identification of stem cell-specific markers for the development of stem cell-specific molecular probes. Such stem cell markers could then be used to develop methods to identify stem cells in tissues and to isolate them directly from tissues. The ability to readily obtain stem cells in human tissues would permit easy harvesting of those cells as well as add greatly to our understanding of tissue cell physiology. An understanding of the mechanisms which control stem cell number can suggest new therapeutic strategies for cancer prevention and treatment, and for reducing morbidity associated with aging.
Accordingly, methods to isolate and expand stem cells from somatic tissue, particularly without significant differentiation, are highly desirable.
Attempts at isolating somatic stem cells have encountered a number of significant difficulties. Attempts at somatic stem cell isolation have been described, for example, in studies to enrich for hematopoietic stem cells (HSCs; Phillips et al., 2000). However, although high degrees of enrichment have been reported, so far HSCs (and other somatic stem cells) have neither been identified nor purified to homogeneity. A major obstacle to these two challenges is the lack of stem cell-specific molecular probes.
Thus, despite the need for methods to isolate somatic cells from an individual, it has not been possible to readily do so. Accordingly, it would be desirable to have a method to identify markers associated with somatic stem cells in mammalian tissues. It would also be desirable to have methods to detect aging in mammalian tissues, including humans. Finally, it would be desirable to have methods to reduce or reverse at least some of the consequences of such aging in adult mammalian tissues, including humans.